The ac impedance spectroscopy was applied to dye-sensitized solar cells with TiO2 or Nb2O5 as the semiconductor electrode. It was found that the impedance spectra consisted of three or four semicircles in the Nyquist representation. The arc with a characteristic frequency of a few hundred Hz was attributable to the electron transport between the semiconductor particles or within the particles, the arc at a few Hz to the electron transport at the electrode/dye/ electrolyte interface, and the arc at the frequency in the order of 10-1 Hz to the reduction of the electrolyte at the Pt electrode/electrolyte interface. The resistance of the arc at a few Hz decreased as the amount of the dye adsorbed on the TiO2 electrode and the irradiation intensity increased. When the Nb2O5 particles with large BET surface area were used as the electrode, the internal resistance of the solar cell was lowered; especially the shrinkage of the arcs in diameter at a few hundred Hz and a few Hz was significant.
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